Device for producing sustained magnetic self-focusing streams



w. H. BENNETT 2,925,505 DEVICE FOR PRODUCING SUSTAINED MAGNETIC SELF-FOCUSING STREAMS Filed April 5, 1958 Feb. 16, 1960 ELECTRON ACCELERA 00 SOURCE 00 SOURCE J 0 32 dj DURATION OF I; INJECTION g PULSE STEADY COMPONENT 3 INTER PULSE 1 INTERVAL l7 23 TI M E INVENTOR WILLARD H. B EN N ETT ATTORNEY}! ni d ta Pat nnvrcrz FOR rnonucmc sUsraiNEn MAG! NETIC SELF-FOCUSING STREAMS Willard H. Bennett, Washington, D.C. Application April 3, 1958, Serial No. 726,299

1 Claims. Cl. 313-62) (Granted under Title 35, US. Code (1952), sec. 266) The invention described herein may be manufactured and used by or for the Government of the United States of America for governmental purposes without the payment of any royalties thereon or therefor.

The present invention relates to electron accelerating devices and more particularly to a device'for producing sustained magnetic self-focusing streams of relativistic electrons in a closed loop.

Heretofore devices have been used for accelerating electrically charged particles such as electrons to high velocity and hence a high potential by means of magnetic induction effects. The electrons are then diverted from their circular orbit to produce a desired result such as bombardnetic guide-field similar in form to that of a cyclotron.

ing a target to produce X-rays. It has also been proposed that a self-focusing stream be produced in a closed loop in a betatron, wherein motions of the electrons in a selffocusing stream in directions transverse to the axis of the stream would produce radiation which would damp such motion and that such damping would oppose the thermal dispersion of the stream. Such a stream would involve the simultaneous acceleration of all of the electrons in the stream and have one major, difficulty. The combined action of the accelerating electric field and the selfmagnetiefield of the current in'the stream drives the electrons toward the axis. The continued application of the accelerating electric force produces a continually increasing density of electrons with their associated space- "charge neutralizing ions near the axis, andthe rate at which energy must be supplied to compensate the loss of energy of the electrons due to their collisions with the ions increases correspondingly. Such a rate may amount to thousands of kilowatts.

' It is accordingly, an object of the present invention to provide a device for producing self-focusing streams of relativistic electrons in a closed loop.

Another object is to provide a device which is capable of holding electrons in a closed loop orbit sufficiently long to. prevent the electrons from escaping from the guide field.

Still another object is to provide a device which is capable of intensifying the current within a closed loop sufficient to produce self-focusing streams.

Other objects and advantages of the present invention will hereinafter become more fully apparent from the following description of the annexed drawings, which illustrates the preferred embodiments, and wherein:

Fig. 1 schematically illustrates an end view of the device of the present invention; 7 i

Fig. 2 illustrates a section through the chamber which illustrates the electron injection tubeand the electron path of the device shown by illustration in Fig. 1;

Fig. 3 is another section through the chamber which illustrates a small coil of wire near the outer surface thereof;

Fig. 4 represents the magnetic field near the small coil at the edge of the discharge chamber; and

Fig. 5 illustrates a modification of the device of Fig. The device of the present; invention comprises an Electrons from an electron linear accelerator are injected in pulses at full energy into the chamber within the;

guide-field. The electrons injected in each pulse are deflected by the bumps in the guide-field and remain long enough for the electrons to radiate energy and shrink in loop radius sufficient to prevent the electrons from escaping from the chamber; By successively injecting electrons into the chamber, the cycling current is built up to the minimum critical value for self-focusing and held there by the guide-field.

Now referring to the drawings wherein like reference characters represent like parts throughout, there is shown by illustration in Fig. l, a device according to the present invention. The device includes a magnetic guide-field similar to that of a cyclotron and which is made to decrease in magnitude with increasing radius at a'slow rate. The rate of decrease of the magnetic field (H) being approximately inversely proportional to r" where r is the radial distance from the axis of the field and n has a positive value less than three-fourths. 'The magnetic field structure is made up from steel laminations or any other suitable material of appropriate contour which includes a pair of cylindrical pole pieces 15 and 16 separated by an air gap within which an annular discharge chamber 17 is placed. Yoke member 18 completes the magnetic circuit for the flux set up in the cylindrical pole pieces. The cylindrical pole pieces. 15 and 16 are surrounded by an annular winding preferably split into two coils 19 which are wound in the same rela tive direction and connected in series for energization from a direct current source 21 to maintain the poles steadily magnetized. The spacing between .the poles provides a space distribution such as to producea stabilizing field on charged particles within a chamber 17 as to confine the particles within said chamber to substantially predetermined circular orbits.

i The chamber 17 is made of any suitable material such as metal, glass, Sillimanite, porcelain, etc., with side walls 22 supported by a support rod 23 across the center of the chamber which prevents the chamber from collapsing when under a high vacuum. The chamber may be held in position by any means familiar in the art, one of which is illustrated by two pair ofsupports 24 on the outer circumferential surface and adapted to be positioned within the magnetic guide-field with the lines of force symmetrical with the axis of the chamber. An electron linear accelerator 25 of any well known type which periodically injects electrons into the chamber through an electron injection tube 26 is located at the midplane of the chamber and arranged tangentially to the inner periphery of the chamber. The tangential arrangement permits the electrons to be injected into the chamber "such that on entering the chamber the magnetic guidefield will force the electrons into an orbit about an axis near the axis of the chamber.

The electron injection tube 26 is surrounded by a magnetic shielding tube 27 which'magnetically shields the beam of electrons from the guide-field untilthe beam emerges from the injection tube and enters the magnetic guide-field which crosses the chamber. The magnetic shield is made of soft iron or like magnetic material and consequently weakens the field at that point to aid in preventing the electrons in the initial loops from hitting the end of the injection tube at 28 while the electrons are rotating within the chamber.

The chamber is provided with a coil 31 near the 'outer surface thereof with axis parallel to the axis of the charm her. The coil is positioned at a pointmore than degrees from the electron injection tube 26. Current from acurrentsource 32 is pulsed through the coil dur- Patented Feb. 16, 1960 ing the electron injection pulse and for an instant thereafter to provide an increased magnetic field within the chamber adjacent to the coil. The magnetic field produced by the coil exerts a magnetic force on the initial electron. orbits of the injected electrons such that at point 33, the direction of motion is deflected away from the surface of the chamber. This slight deflection coupled with the effects of the locally reduced magnetic field near the end of the injection tube produced by the magnetic shielding about the electron injection tube operates to prevent the initial orbits of the injected electrons from hitting the end of the injection tube. The field set up by the current in the coil and the magnetic shielding tube wil'l be referred to as the piler field. After the end of the injection pulse, the current in the small coil 31 is removed, and the injected electrons will follow a path which has somewhat the shape of an apple at the bottom where the electrons are deflected by the bump in the piler field.

In operation of the device, the cylindrical discharge chamber 17 is evacuated and positioned in the radially decreasing magnetic field such that the lines of force of the magnetic guide-field are symmetrical with the axis of the chamber. High energy electrons from the electron linear accelerator 25 are injected in pulses into the chamher at an energy large compared to the rest-energy of an electron wherein the electrons are directed into orbits by the guide-field. Simultaneous with the injection of electrons, current is applied to coil 31 to increase the magnetic field in the chamber directly opposite to the coil at 33 for a period which is slightly greater than the electron injection pulse; The current applied to' and passed through the coili simultaneously with the beginning of the electron injection pulse is applied for a short time after the ending of the electron injection pulse and sets up a magnetic field which is strongest near the surface of the chamber. This added magnetic field acts upon the initial orbits of the injected electrons such that the guiding centers of the orbits are shifted into the vicinity of the axis of symmetry of the regular guide-field produced by magnetic poles 15 and E6. The sudden rise in the magnetic guide-field opposite the coil tends to produce a slight shift in the initial orbits at that point to change the orbits enough to prevent the particles from hitting the end of the injection tube at 23. The field produced by the coil 31 is weaker toward the axis of the tube'and.

aeaatots-- 4" rent i for self-focusing which the relation can be determined from where c is the speed of light; is-tbe charge on the elecx is the energy of a freshly injected electron and x is the energy of the'electron after a time, 't.

Representing the mean angular divergence at injection as w, the critical current i after the electrons have decreased in energy to x from the value at injection of x is determined by angular divergence at injection of one-tenth milliradian, if slowed to 11.4 mev. must build up to a current of 4600 amperes to become self-focusing.

therefore does not interfere with orbits circling near to the axis with-in the guide-field. After the pulse applied to the'coil is suddenly decreased tozero, the particles move only under the influence of the reduced magnetic field adjacent to the shielding tube for the entering stream and the magnetic field produced across the chamber by poles 15' and 16.

g The motion of electrons in approximately circular loops results in the electrons radiating some of their energ'y', and the'reduction in energy produces a corresponding reduction in loop radius such that the particles remain within the chamber and are held spiraling in space about the axis away from the walls of the chamber. The injection of pulses of electrons and pulses of current to the coil near the surface of the chamber are regularly repeated for continued use of the device.

The modification shown by Fig. 5 illustrates a device according to that of Fig. 1 except the coil 31- is positioned along the outer surface of the chamber rather than within the surface. This modification is the same as for that offig. 1 in operation and follows the description as pointed out above with respect to the device shown in Fig. 1. However, with coil: 31 on the outside of the chamber, the chamber must be made of insulating material.

The first phase build-up of current in the orbits spiralmg inward toward the"- center of the guide-field continues until the tot-at current exceeds the minimum criticatcur- As soon as the critical current is exceeded, the moving electrons will begin to draw together into a self-focusing stream under the efiects of the self-magnetic field pro: duced by the electron travel, and the acceleration of the electrons by the self-magnetic field of the stream make the electrons radiate energy of motion transverse to the direction of the stream. This is to be distinguished from the radiation by the electrons due to their acceleration in the guide field.

The electrons are injected into the chamber at full energy and accumulate in the stream without being further energized before the critical current for self-focusing is reached. An application of steady-state self-focusing stream is the use of the stream as a strong magnetic guide-field for ions while the ions are being accelerated to very high energies by means with which the electrons in the stream are not resonant. l

ions can be injected into the stream to run in the opposite direction around the loop. The ionscannot be stored in the stream by any practical kind of process using radiation like that which was usedfor storing electrons of the stream. This kind of ion injection can be made rapid enough to keep the stream filled with high energy ions of the species being injected and thus prevent the ions formed by ionization of residual gas from remaining in the stream and forming any importantpart of the stream.

Obviously many modifications and variations ofthe present invention are possible in the light of the above teachings. It is therefore to be understood that within the scope of the appended claims the invention may be practiced otherwise than. as specifically described.

What is claimed is:

l. in a device for producing self-focusing streams of particles comprising a chamber adapted to be evacuated and positioned. within a magnetic guide field with the lines of force symmetrical about the axis of said chamber, means for injecting periodic pulses of high speed particles into said chamber near the inner surface thereof,

means associated with said injection means for magnetisurface of said chamber toincrease the magnetic field V for passing suddenly rising current pulses through said electromagnetic means, said pulses being applied just prior to the start of each of said electron injection pulses and for a longer duration.

2. In a device for producing self-focusing streams of particles comprising a chamber adapted to be evacuated and positioned within a steady magnetic guide field with the lines of force symmetrical about the axis of the chamber, a coil of wire positioned near to the outer surface of said chamber and adjacent thereto with the coil axis parallel with the chamber axis, means for injecting periodic pulses of high speed particles into said chamber near the inner surface thereof, means associated with said injection means for magnetically shielding said high speed particles on entering said chamber, means for passing suddenly rising current pulses through said coil of wire to increase the magnetic field across said chamber adjacent to said coil of wire, said pulses passing through said coil being simultaneous with said periodic pulses of high speed particles and for a period slightly longer than the pulse period of said particle injection.

3. In a device for producing self-focusing streams of electrons comprising a chamber adapted to be evacuated and positioned within a steady magnetic guide field having a field intensity which decreases slightly in magnitude with increase in radius with the lines of force symmetrical about the axis of said chamber, an electron linear accelerator positioned tangentially to the inner periphery of said chamber for injecting periodic pulses of high speed electrons into said chamber, a magnetic shield surrounding said stream of electrons entering said chamber from said electron linear accelerator which locally decreases the magnetic field across said chamber in the vicinity of said electron accelerator, a coil of wire positioned near to the outer surface of said chamber with the coil axis parallel with the chamber axis, means for passing suddenly rising current pulses through said coil of wire to locally increase the magnetic field periodically in the vicinity of said coil, said pulses through said coil being applied simultaneous with said pulses of high speed electrons and for a duration slightly longer than the pulse period of said electron injection, said local de crease in magnetic field about the electron stream entering from said electron accelerator and said increase in mag netic field in the vicinity of said coil acting upon initial orbits of electrons to force said electron orbits closer to the axis of said chamber.

4. A device as claimed in claim 3 wherein said steady magnetic field decreases radially approximately inversely proportional to r where r is the radial distance from the axis of the field and n has positive values between Vs and /1.

5. A device for producing self-focusing streams of charged electrons which comprises means for producing a steady magnetic guide field having a field intensity which decreases slightly in magnitude with increase in radius, a chamber adapted to be evacuated and positioned within said magnetic field with the lines of force symmetrical about the axis of the chamber, an electron linear accelerator positioned tangentially to the inner periphery of said chamber for injecting periodic pulses of high speed electrons into said chamber, a magnetic shield surrounding said stream of electrons entering from said electron linear accelerator which locally decreases the magnetic field across said chamber in the vicinity of said electron accelerator, a coil of wire positioned near to the inner surface of said chamber with the axis perpendicular to a radius and parallel to the axis of said chamber, means for passing suddenly rising current pulses through said coil of wire to locally increase the magnetic field periodically in the vicinity of said coil, said pulses through said coil being applied simultaneous with said pulses of high speed electrons and for a duration slightly longer than the pulse period of said electron injection, said local decrease in magnetic field near the shield about said electron stream from said electron accelerator and said increase in magnetic field in the vicinity of said coil acting upon initial orbits of electrons to force said electron orbits toward the axis of said chamher.

6. A device as claimed in claim 5 wherein said steady magnetic field decreases radially approximately inversely proportional to r" where r is the radial distance from the axis of the field and n has /s and A.

7. A device for producing self-focusing streams of charged electrons as claimed in claim 5 wherein said coil of wire is positioned on the outer surface of said evacuated chamber.

positive values between References Cited in the file of this patent UNITED STATES PATENTS 2,193,602 Penney Mar. 12, 1940 2,242,888 Hollman May 20, 1941 2,830,211 Kaiser Apr. 8, 1950 2,812,463 'Teng Nov. 5, 1957 FOREIGN PATENTS 682,504 Great Britain Nov. 12, 1952 

